1. Field of the Invention
This invention relates to optical master oscillator power amplifiers (MOPAs), and more particularly to MOPAs in which an amplified return beam is isolated from the master oscillator which generated the input beam.
2. Description of the Related Art
MOPA laser systems are used to generate high power laser beams. Such systems are described in U.S. Pat. Nos. 4,734,911 to Bruesselbach and 4,757,268 to Abrams et al., both assigned to Hughes Aircraft Company, the assignee of the present invention.
A known MOPA system is shown in FIG. 1. A master oscillator 10 is provided in the form of one of several known lasers, using a gas-dye or solid-state laser medium to produce a pulsed laser beam 12. The laser beam is coupled into an optical amplifier 14, and the amplified beam is then forwarded to a phase conjugate mirror (PCM) 16. The latter element returns the beam with a wavefront reversal back through amplifier 14, where it is amplified a second time. Stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) PCMs have been employed for this purpose. Processing by the PCM and double passage through the amplifier results in a negation of amplifier-induced aberrations in the beam, in a known fashion. Examples of such aberrations are spatial thermal differences for a solid amplifier medium, and turbulence for a gaseous amplifier medium.
It is important that the twice-amplified beam be coupled out of the system without re-entering the master oscillator (MO). An optical polarization scheme has been used to achieve this isolation. The beam 12 emitted from the MO 10 is polarized, as indicated by the horizontal polarization symbol 18; the arrow below this symbol indicates the direction of beam travel. The polarized beam 12 is directed onto a polarization sensitive beam splitter 20, which transmits the polarized beam to amplifier 14. From there it is transmitted through a quarter-wave plate 22 to PCM 16, and reflected back through quarter-wave plate 22 for a second amplification. Quarter-wave plate 22 rotates the polarization of the conjugated wave by 90 on its return path from PCM 16; this rotated polarization is indicated by vertical vector arrow 24. The polarization sensitive beam splitter 20 deflects the vertically polarized return beam along an output path 26, which is directed away from the MO 10. This reflected beam constitutes the amplified output of the system. A magneto-optic isolator 28 may be placed in the output path 12 from MO 10 to block any horizontally polarized light remaining in the return beam, in case full polarization rotation has not been achieved.
A principal limitation of this type of system is that the presence of the polarization elements significantly limits the beam power that can be handled, and requires dual polarization operation of the power amplifiers. The diameter of the beams that can be accommodated is also quite restricted by the limited available size of quarter-wave plates. Magneto-optic isolators are also limited to low power regimes, and require bulky magnets.